1. Field of Invention
This invention relates to lighting systems for vision systems.
2. Description of Related Art
The light output of any device is a function of many variables. Some of the variables include the instantaneous driving current, the age of the device, the ambient temperature, whether there is any dirt or residue on the light source, the performance history of the device, etc. Machine vision instrument systems typically analyze features or objects within their field of view using methods which may determine, among other things, the location of edges of a feature on an object or the contrast within the region of interest where the features or objects may be found. To some degree, this determination is affected by the character of incident light or transmitted light.
A machine vision system programmer or operator often wishes to illuminate a workpiece to achieve a specific image characteristic. The image characteristic may be a specific average gray level value in a region of interest or a goal such as to maximize the difference in average gray level between various regions of interest, or to maximize a gradient within a region of interest. More generally, the desired characteristic may be a complex series of goals defined across an image scan line.
In many applications, the relationship between the imaging subsystem and the workpiece under inspection is predictable. In such applications, the predictability of the situation allows a simple form of reproducible lighting control. As illustrated in U.S. Pat. No. 5,753,903 to Mahaney, for example, closed-loop control systems are used to ensure that the output light intensity of a light source of a machine vision system was driven to a particular command level. These conventional closed-loop control systems prevent the output light intensity from drifting from the desired output light intensity due to variations in the instantaneous drive current, the age of the light source, the ambient temperature, or the like. Accordingly, it is possible to determine a light intensity setting or level for a single, spatially fixed illumination source and an actual workpiece. However, even when such a simple form of reproducible lighting control is possible, performing a search for an optimal light source setting can become problematic because each of the steps of light adjustment, acquisition of each video frame, and evaluation of each video frame requires a finite period of time. In particular, common halogen lights require a lengthy period of time to stabilize their output after their current drive is altered. Thus, although there are several conventional approaches to choose a satisfactory light intensity setting for a given workpiece and a single fixed light source, there may be some associated delays which reduce the available throughput of the associated vision system.
In U.S. application Ser. No. 09/736,187, which is incorporated herein by reference, discloses a vision system and method for determining a prescribed illumination setting for a vision system that has a controllable lighting system. The prescribed illumination setting is usable to acquire a desired image that has a desired image characteristic in a region of interest. As disclosed in the '187 application, the method comprises obtaining a plurality of base images of at least the region of interest, where each base image comprises an actual image corresponding to an actual illumination setting, and determining, for at least the region of interest, a synthetic image based on a current prospective illumination setting and at least one of the base images. Then, the process evaluates whether the synthetic image corresponds to a desired image. If the synthetic image result does not correspond to the desired image, the process modifies the current prospective illumination setting. The process then repeats the determining, evaluating and modifying steps until at least one synthetic image corresponds to the desired image.